1. Field of the Invention
The present invention relates to a directional switching device for a feeder in a machine tool, and more particularly, to an improved directional switching device for a feeder in a machine tool driven by a direct current motor.
2. Prior Art
Generally, oil pressure cylinder type devices are employed for driving feeders to move machine axles, work tables and the like to prescribed positions. The oil pressure type cylinder device has several advantages such as relative ease of control and great driving power which makes it suitable as a thrust generator device in a feeder. However, its drawbacks include unstable feeder control caused by temperature variations and the use of compressive fluids, which affect productivity and durability of the machinery. In addition it presents such environmental problems as oil leaks and noise.
As opposed to conventional oil pressure cylinder devices, a motor driven feeder method has been proposed. However this method does not completely satisfy the advantages derived by the oil pressue cylinder method in that, particularly with respect to alternating current (A.C.) motors, simple changes in rotational speeds are difficult to achieve which necessitates expensive and complex gear boxes and the like to be installed. Another drawback is the difficulty in securing and maintaining a prescribed stopping position of the feeder which causes inaccurate forward and backward movements and mechanical friction which results in a shorter machine life.
As opposed to these drawbacks associated with the A.C. motor, the direct current (D.C.) motor is more favorable, although it too has disadvantages such as a complex circuit structure for switching the rotational direction, i.e., the direction in which the feeder moves. In the conventional D.C. motor the D.C. current is supplied to the motor's armature by a D.C. bridge made up of four transistors. By a switch controlling the transistor bridge circuit, flow of the armature current is reversed thereby switching the rotational direction. Thus, the structure of the control circuit in the conventional D.C. motor driven device is not only more complex but also requires a protective circuit in order to control any abnormal excessive voltage that is generated when the circuit is stopped for reversing action. Because of this and other drawbacks the D.C. motor has been employed only in special machines such as NC machinery which is equipped with expensive control devices.